Syntactic foam layers and products have been conventionally made by intermixing glass microspheres with a heated liquid binding resin, and at times including reinforcing elements such as fibers. The use of resin in a fluid state heated to effect a cure, however, results in a considerable release of fumes and liquid vapors which can interrupt or destroy the integrity of the material and often must be treated as environmentally objectionable. In contrast heat curing or setting of heated powdered resin in a mixture with microspheres results in little effluent of fumes or liquid vapor. In other words, phenolic resins, for example, during heating to a liquid phase and curing give off vapors in a considerable amount whereas dry phenolic powders during heat curing give off little vapor. In addition, resin in a fluid condition is less adaptable to providing a syntactic foam microsphere mixture capable of the wide range of light densities, weights and strength properties possible by use of resin powder according to the present invention.
A number of product properties can be imparted to a product by use of a reactive resin in powdered form that cannot be accomplished with resin in a liquid form. For example, the bulk density of powdered resin is much less than the bulk density of solid resin of the same composition or liquid formed of the resin by heating it to a melted condition. An important advantage of using powdered resin mixed with ceramic microspheres to form a syntactic foam layer according to the present invention is that the bulk density of the final product can be the same as that of the initial compacted mixture. During heating of the mixture, the powdered resin is converted to melted droplets which can flow over and join the microspheres of the mixture sacs may be aided by an adhesion promoting coating, for example a coupling agent, on the microspheres. The mixture is thus in a sense coalesced into a mass which upon hardening provides a light weight solid foam layer. The foam layer includes closed voids containing evolved gas and air. The bulk density of syntactic foam material as in the present invention can be half that of a conventional material made from liquid resin with intermixed microspheres. Additionally, the present invention eliminates the problems of high viscosity when many microspheres are added to a liquid resin, and eliminates the process problem of getting microspheres (which tend to float) wetted out and incorporated into a liquid resin.
It has been found that finer powder resins function much more effectively to produce the desired results than coarse powders. When the powders included are too coarse, the melting and dispersion of the material between the microspheres become more difficult and are much less effective in providing a uniformly integrated syntactic foam product.
The mixture can be heated by a number of techniques including induction heating with high frequency energy or conductive heating such as with heated platens on opposite major surfaces of the layer of microspheres and resin powder. During heating of a platen in contact with a layer of intermixed powdered resin and microspheres, a skin layer can be formed at the surfaces of the foam. To prevent the mixture from sticking to such platens a layer of separator material, a material which will not bond or unite with the syntactic foam layer, is provided between each of the platen surfaces and the surfaces of the microsphere mixture.
When it is desired that the density of the product be greater than that attained with the pressureless application of heat, the mixture can be compacted by bringing the heating platens closer together to establish a predetermined thickness. The thickness can be established by placing spacer members of the predetermined desired thickness between the platens which will limit the closeness to which the platens can be moved and thereby establish the desired thickness of the product produced. The spacers can also function to limit the area over which the microsphere-resin powder mixture can spread. In other words, they can be used to confine the area of the mixture, such as in a tray, to establish the predetermined dimensions of the product to be produced.
As described herein the invention is used to form a stratum or core for structural laminate panels, but variations of the mixture of glass microspheres and resin powder can be engineered for a wide range of products as well, such as flooring, ducts, and three dimensional products useable for aircraft, trucks, automobiles, ships, boats, industrial tanks and the like. The desired light weight and strength properties of the foam are attained in part by utilizing microspheres or bubbles, preferably hollow ceramic microspheres such as of glass commercially available in various diameters and wall thicknesses. The microsphere diameters and wall thicknesses are selected to impart specific predeterminable shear and compression strengths as well as desired weights and densities when integrated with the resin intermixed therewith. The powdered resin intermixed with the microspheres or bubbles is of substantially finer dimension than the bubbles thereby enabling thorough distribution of the powder and filling of the interstices between the bubbles.
The term "cured" or "curing" as applied to thermosetting resin herein refers to heat processing to a fluid then to a more stable hardened or set condition, but to facilitate description of the invention also refers herein to hardening of thermoplastic resins to a set condition upon cooling after being heated to a fluid condition according to the concepts of the present invention.
The bubbles of the mass may be of different sizes which permits their close compaction into an intimate mass for strength, while the finer resin powder fills the interstices more readily to effect an inter-bonding of the bubbles and resin. The amount of resin incorporated in the core can be just sufficient to effect the desired inter-bonded relation between components of the mixture, which with a light concentration of the powder in the mixture can result in the cured syntactic layer being porous and permeable. More desirably, however, for most applications the concentration of powder in the mixture is such that the cured integrated mass is substantially impermeable to moisture beside having high shear and compression strengths.
The foam mixture can additionally include reinforcing elements such as glass or carbon fibers or fibers of other high strength material. In this regard the fibers may be incorporated in the mixture as individual fibers, as bundles of chopped strands, or as continuous filaments in nonwoven mats or woven fabrics. Other reinforcing elements such as honeycomb structures can also be incorporated in the core material as well.
An object of the invention is to provide a mixture of components for formation of a syntactic foam material capable of being engineered and manufactured economically into products having a wide range of predeterminable structural properties.
Another more specific object of the invention is to provide a basic, easily processed, economically producible, light weight core material capable of providing structural properties in layer form adaptable to use in sandwich structure composites.
A feature of the invention is that the combination of powdered phenolic material and glass microspheres gives off little or no volatiles or fumes during cure.
Another important feature is that the mixture can, within a wide range, be pre-engineered for a desired density, shear strength, compression strength, low flammability and low smoke and high moisture resistance while at the same time being capable of production at a low cost.
A further feature is that a syntactic foam core layer for laminar structures can be produced with no reduction in thickness dimension of the layer during heat processing.
Strength and density of the sheet can be modified by appropriate selection of the size and wall thickness of commercially available microspheres and the size and type of powder resin particles. The microspheres can be provided with a coating of a coupling agent such as silane to facilitate their coverage and inter-bonding by the melted resin powder. As pointed out the resin powders are of finer size than the small microspheres and fit in the interstices of the mass of microspheres to effect an inter-bonding upon heating and hardening. As the amount of resin present increases as part of the mixture the greater the weight of the microsphere foam product becomes.
The process of producing the syntactic sheet or core material involves first combining the microspheres and powdered resin. The combination is deposited as a layer on a base having a release agent or commercial release film over its surface to prevent bonding to the final foam product. The base also is provided with boundary means to define the dimensions of the product. The combination is first vibrated for thorough intermixture of the resin powder in the microsphere interstices, and the mixture is then heated to melt the resin particles for inter-bonding with the microspheres. Although pressure is not necessary to effect the inter-bonding of microspheres and resin, pressure can be applied to the mixture as an assist in effecting its compaction when a more dense product is desired. Such compaction of the mixture can be effected by bringing the overlying and underlying heated platens to a preselected spacing for a desired thickness of the layer. The thickness of a given deposited layer as well as its density and mechanical properties of the final product can thus be predetermined.
As indicated, in addition to a mixture of microspheres and resin powder alone providing the base mixture for the product, structural reinforcement elements such as fibers (hollow or solid) or fiber bundles can also be selectively included. Other reinforcements for example can be mats of random reinforcing fibers or reinforcing fiber fabrics, both woven and non-woven, rods, glass flakes and honeycomb structures which in appropriate locations can improve mechanical properties such as shear strength and shear modulus of the sheet product.
Resin in powder form such as can be formed by grinding down solid resin, is selected for its fineness to fill the interstices of the microspheres which can be accomplished more readily than with coarser powders. To provide the binding relation for integration of the mixture, the powders are also selected for their chemical reactivity and heat softenable adhesive affinity for the glass microspheres. They can also be selected for low flame and smoke properties. In this regard, the invention is quite versatile in permitting trial and error establishment of the engineered properties desired.
The resulting syntactic foam product is corrosion resistant, electrically and thermally relatively non-conductive, non-magnetic, electromagnetically transparent, light weight, much less than the weight of steel, has high strength and dimensional stability, and is adaptable to providing a wide range of physical and mechanical properties.
Any of a number of reactive resin powders may be used to provide the specific desired properties including, but not limited to, phenolic resins aforementioned as well as epoxy and epoxy-modified phenolic resin, polyester resin powders, polyurethane, and polyphenylene sulphide. In addition powder resin from waste dust collection devices, such as in a resin manufacturing plant, can be used in the present invention. Disposal of these waste materials is particularly an environmental problem because they are most frequently reactive powders. The present invention is thus additionally advantageous in that it can frequently eliminate environmental problems by providing a value-added use for waste materials.